The following is a brief description of a channel estimation method and pilot signals.
To detect a synchronous signal, a receiver should have information regarding wireless channels such as attenuation, phase shift, or time delay. Here, the term “channel estimation” refers to estimation of the reference phase and the size of each carrier. Wireless channel environments have fading characteristics such that the condition of a channel irregularly changes in the time and frequency domains as time passes. Channel estimation serves to estimate the amplitude and phase of such a channel. Namely, channel estimation serves to estimate a frequency response of a wireless link or a wireless channel.
In one channel estimation method, a reference value is estimated based on pilot symbols of several base stations using a two-dimensional channel estimator. Here, the term “pilot symbols” refers to symbols that do not contain actual data but instead have high power to support carrier phase synchronization and acquisition of base station information. The transmitting and receiving ends can perform channel estimation using such pilot symbols. Specifically, the transmitting and receiving ends estimate a channel using pilot symbols known to both the transmitting and receiving ends and reconstruct data using the estimated value.
FIG. 1 illustrates an example of a general pilot structure used in a single-transmit-antenna structure.
The pilot structure of FIG. 1 is applied when one transmit antenna is used. When one antenna is used, two pilot subcarriers are used for each even symbol and two pilot subcarriers are used for each odd symbol. In this case, an overhead of about 14.28% may occur due to pilot subcarriers.
FIG. 2 illustrates an example of a general pilot structure used in a two-transmit-antenna structure.
In downlink, Space-Time Coding (STC) is used to provide high-order transmit diversity. Here, two or more transmit antennas are needed to support STC.
As shown in FIG. 2, two transmit antennas (first and second antennas) can simultaneously transmit different data symbols. Here, data symbols are repeatedly transmitted in the time domain (space-time) and the frequency domain (space-frequency). Accordingly, the pilot structure of FIG. 2 can exhibit higher capabilities when transmitting data although receiver complexity is increased.
The method of allocating data in the example of FIG. 2 can be changed in order to use two antennas having the same channel estimation capabilities. A respective pilot symbol is transmitted twice through each antenna. The position of the pilot symbol is changed over four symbol durations. Symbols are counted starting from the beginning of the current region, and the first symbol number is even.
In the example of FIG. 2, pilot subcarriers are used for channel estimation. Here, an overhead of about 14.28% may occur due to pilot subcarriers.
FIG. 3 illustrates an example of a general pilot structure used in a four-transmit-antenna structure.
When four antennas (first, second, third, and fourth antennas) are used, transmit diversity can be improved, compared to when two antennas are used. Even when four antennas are used, the pilot structure of FIG. 3 can exhibit the same channel estimation capabilities as when two transmit antennas are used.
As shown in FIG. 3, respective pilot channels of the antennas are allocated to each symbol. For example, when one symbol includes 14 subchannels, respective pilots of the four antennas are allocated to subcarriers of each symbol. Thus, an overhead of about 28.57% may occur due to pilot subcarriers.
As described above, an overhead of about 14.28% may occur due to pilot subcarriers when one transmit antenna is used and when two transmit antennas are used. In addition, an overhead of about 28.57% may occur due to pilot subcarriers when four transmit antennas are used.